Page 172 - Multifunctional Photocatalytic Materials for Energy

158 Multifunctional Photocatalytic Materials for Energy

etching, and the hydrothermal method. Among these techniques, the hydrothermal
method is the one most widely used to fabricate high-quality TiO 2 NRs/NWs because
of its low cost, its easy operation, the simplicity of the process and the equipment
used in the process, and its suitability for mass production of nanomaterials. To ob-
tain high-ordered TiO 2 NWs/NRs on transparent conducting substrates (TCSs), a thin
compact TiO 2 layer is usually utilized as a seed layer to grow monocrystalline arrays.
Grimes et al. first grew crystalline TiO 2 NW arrays directly onto transparent conducting
oxide (TCO) substrates via a nonpolar solvent/hydrophilic substrate interfacial reaction
under mild hydrothermal conditions [17]. In this process, a compact TiO 2 layer ~20 nm
thick was coated on the F-doped SnO 2 (FTO) glass by immersing it in a 0.2 M TiCl 4
solution before growing the NWs. Such a layer not only prevented electrical shorting in
DSSCs but also enhanced the density, and ultrafine square NWs ranging from 10 nm to
35 nm were obtained, as shown in Fig. 8.4. TiO 2 NWs/NRs can also be grown on FTO
glass without seed layers. Aydil and Liu developed a facile, hydrothermal method for
the first time to directly grow oriented, single-crystalline rutile TiO 2 NR films on FTO
substrates (see Fig. 8.4) [18]. A seed layer is not necessary because there is a small
lattice mismatch between the FTO substrate and rutile TiO 2 , which plays a key role in
driving the nucleation and growth of the rutile TiO 2 NRs on FTO.

8.2.1.3 Fabrication of TiO 2 NTs
TiO 2 NTs, which are usually fabricated by electrochemical anodic oxidation, are
widely used in thin-film solar cells, especially DSSCs/QDSCs because of their
highly ordered structure and large surface area. Anodic oxidation is operated under a
two-electrode system, in which Ti foil acts as an anode and Pt foil acts as a cathode.
The anodic solution usually contains organic solvents (ethylene glycol or acetone)
−
and F ions. When the voltage is applied to this system, Ti foil is etched into a highly
ordered tubular structure. The length and diameter of the TiO 2 NTs can be controlled
−
by adjusting the react-time, reaction voltage, concentration of F ions and water, and
viscosity of the solution. Zwilling and colleagues first reported this technique in 1999
in regard to the anodization of Ti in chromic acid/HF mixtures [19]. However, al-
though a highly ordered tubular structure was successfully obtained, the thickness of
these first-generation NT films were very limited (<500 nm). To address this problem,
Macak et al. introduced two strategies to improve the length of the NTs: (i) use of neu-
tral electrolytes that allowed, under suitable conditions, significantly longer tubes to
be grown; and (ii) use of nonaqueous electrolytes that significantly improved the tube
length and wall roughness (smooth, high aspect ratio NTs) [20–22]. Use of such an
anodization procedure in nonaqueous electrolytes produces very thick TiO 2 NT layers
(several 100 mm) with very smooth walls, which can be grown to show an almost ideal
hexagonal arrangement [23,24].
The hydrothermal method is another way to obtain TiO 2 NTs. Ding and Guo et al.
fabricated rutile TiO 2 -tapered NTs with rectangular cross sections via anisotropic cor-
rosion of rutile TiO 2 NRs [25,26]. Fig. 8.5 shows a possible mechanism for the forma-
tion of rectangular TiO 2 NTs. When the rutile NRs were treated with HCl during the
hydrothermal process, the HCl preferentially etched the TiO 2 NRs toward the [001]
direction because the (001) facet of rutile is much more reactive than the side-wall